This invention relates an LED print head which is used for example in an electrophotographic printer or copier, and in which light-emitting diodes (LEDs) for forming images are aligned in rows.
An LED unit in which a plurality of LED array chips are arranged in a straight line, and driver chips are also provided is known as a light source for forming images (exposure), in printers or copies using electrophotography, and an LED print head which is an elongated semiconductor device wherein a linear rod lens array which collimates light beams output from the light-emitting parts of the LED units respectively to predetermined positions on an electrophotographic photosensitive drum, is integrated with an LED unit, is known as an exposure device.
Also, in an example of a light-emitting diode which does not require an AlGaAs thick film layer, a light-emitting part comprising a plurality of epitaxial layers is formed on the upper side of a substrate, a partial electrode is formed in the center of a front surface of the epitaxial layer while a whole-surface electrode is formed on the lower surface of the substrate, the substrate and the plurality of epitaxial layers forming the light-emitting parts are spatially connected in the center, and a gap layer is formed between the substrate and the epitaxial layers except at this connecting part (see for example Japanese Patent Kokai Publication No. H7-235690 (p. 3-5, FIG. 1).
However, for example, in LED array chips 312 of an LED print head 310 of the prior art shown in FIG. 40, individual electrode pads 312c connected via individual electrodes 312b to respective light-emitting parts 312a as shown in FIG. 41 are required to have a relatively large surface area (e.g., 100 μm×100 μm) for wire bonding. Also, as shown in FIG. 40, the same number of individual electrode pads 312c are required as the number of connecting wires 314 between the LED array chips 312 and driver IC chips 313, so it is difficult to reduce the surface area of the LED array chips 312, and to make the LED array chips 312 compact.
Likewise, individual electrode pads for connection via wires 314 to LED array chip 312, as well as electrode pads for connection via wires 315 to a printed circuit board 311 must also be provided on the surfaces of driver IC (integrated circuit) chips 313. As shown in FIG. 40, the same number of individual electrode pads are required as the number of the connection wires 314 between the two chips, and the same number of electrode pads as the number of connection wires 315 between the driver IC chips 313 and the mounting substrate 311 are required, so it is difficult to reduce the surface area of the driver IC chips 313 and to make the driver IC chips 313 compact.
Also, when an optical fiber array on a base plate is used, electrodes are formed on a plurality of light-emitting diode array chips and corresponding electrodes are formed in circuit conductors, so it is difficult to reduce the surface area of the diode array chips and circuit conductors, and to make them compact.
For the same reason, when it is attempted to reduce the chip width to reduce the material costs, there is a limit in the amount of chip width reduction due to the provision of the electrode pads, and it is difficult to substantially reduce the material cost of the LED array chips 312 and driver IC chips 313.
Considering now the proportion of the light-emitting areas and other areas in a device having LED array chips 312 and driver IC chips 313, the individual electrode pads 312c occupy a larger area than the light-emitting parts 312a, so the proportion of material required to form electrode pads such as the individual electrode pads 312c which are non-light-emitting areas is large, and the proportion of material required for surface area functioning as light-emitting areas, is small. This is ineffective from the viewpoint of material usage efficiency. Moreover, even if it is attempted to improve material usage efficiency, as long as electrode pads are provided on the chips, material is required for the area occupied by the electrode pads, and a significant improvement of material usage efficiency is difficult to accomplish.
The thickness of the LED array chips 312 has to be adjusted to a thickness (e.g., approximately 300 μm-350 μm) equivalent to that of the driver IC chips 313 for easier handling of the chips during die bonding and to prevent problems such as short circuits between wires and chips, and to facilitate wire-loop formation during wire bonding. In an example shown in FIG. 42, a GaAsP epitaxial layer 324 is formed on a GaAs substrate 325, and a Zn diffusion region 321 is formed in part of the GaAsP epitaxial layer 324. An individual electrode 322 is provided on the Zn diffusion region 321, an interlayer insulation film 323 is formed except at the vicinity of the individual electrode 322 on the GaAsP epitaxial layer 324, and a rear electrode 326 is formed underneath the GaAs substrate 325. The Zn diffusion region 321 which forms a pn junction has a depth of approximately 3 μm-5 μm from the surface of the GaAsP epitaxial layer 324, but the GaAsP epitaxial layer 324 is formed as thick as approximately 50 μm-100 μm to decrease the defect density in the area forming the pn junction, and the GaAs substrate 325 is formed to have a thickness of approximately 250 μm-300 μm in order to maintain ease of chip handling.
However, the region which functions as light-emitting region is approximately 3 μm-5 μm of the Zn diffusion region 321, far smaller than the thickness of the LED array chip 312 which is approximately 300 μm-350 μm, so if the thickness of the LED array chip 312 is made equivalent to that of the driver IC chip 313, it is uneconomical from the viewpoint of material usage efficiency. Also, from the viewpoint of the light-emitting function, although the GaAs substrate 325 merely supports the GaAsP epitaxial layer 324 which provides the light-emitting function, and the GaAs substrate 325 itself is not involved in the light-emitting function, a thickness of several hundred μm is required for ensuring a sufficient yield rate in the production of the support and wire bonding. As it is difficult to reduce the thickness, it is difficult to reduce materials and cut down on material costs.
The amount of semiconductor material can be reduced by forming semiconductor thin film pieces of LED array chips on a first semiconductor substrate, and transferring and bonding the semiconductor thin film pieces onto a second semiconductor substrate. By this method, the LED array chips can be made thinner than the GaAs substrate. However, the parts (bonding layers) of the second semiconductor substrate where the semiconductor thin film pieces are to be bonded have to be higher than other parts for ease of bonding.
Also, if the bonding layers are made from a noble metal, since the bonding layers must be higher than the other parts as described above, the material cost of the bonding layers is increased. The amount of time required for forming the bonding layers is also substantial.
In addition, the arrangement pitch of the semiconductor thin film pieces of the LED array chips formed on the first semiconductor substrate is typically different from the arrangement pitch of the driver IC chip areas formed on the second semiconductor substrate, so it is necessary to have the thin film pieces conform with (or converting their pitch into conformity with) the arrangement pitch of the driver IC chip areas for the LED array chips, and thereafter bond a plurality of semiconductor thin film pieces, while simultaneously supporting them. This however is difficult.